1. Field of the Invention
The present invention relates generally to sintered tungsten carbide-containing materials and methods for making same and, more particularly, to sintered tungsten carbide-containing materials containing substoichiometric levels of carbon and methods for making same.
2. Description of the Related Art
Sintered tungsten carbide-containing materials are popular in applications requiring high wear resistance and/or cutting performance. To achieve the desired levels of strength or toughness and enhance the sinterability of the tungsten carbide-containing materials, metal binders are often added to the starting materials prior to sintering. The resulting sintered tungsten carbide-metal materials are referred to as tungsten carbide cermets or hard metals. Cobalt is known to be the best binder for tungsten carbide, however, careful control of the level of carbon in the tungsten carbide-containing material before sintering must be exercised or inferior material properties result.
If the carbon in the pre-sintered material is substoichiometric relative to the level of tungsten, an undesirable brittle complex intermetallic carbide, specifically, eta-phase Co.sub.3 W.sub.3 C, is easily formed and remains in the sintered product. If the carbon in the pre-sintered material is hyperstoichiometric relative to the level of tungsten, undesirable graphite or amorphous carbon precipitates and remains in the sintered product. Total carbon is, therefore, typically controlled in tungsten carbide/cobalt cermets to a difference from stoichiometric of within 0.1% (i.e., +/-0.05%) to produce acceptable material.
While tungsten carbide-cobalt cermets find tremendous utility, there are situations that demand higher hardnesses and better wear resistances than that which is achieved with the traditional tungsten carbide-cobalt cermets. Higher hardnesses and improved wear resistances can be achieved by substantially reducing or eliminating the metallic binder. However, the sinterability of these "binderless" materials is greatly hindered.
To produce an object formed of sintered tungsten carbide-containing material, a preform containing tungsten carbide powder is formed, which is usually subjected to an organic binder burn-out process, and then sintered. Unfortunately, the organic binder provides excess carbon which is often introduced into the pre-sintered tungsten carbide-containing material through the use and subsequent pyrolysis of organic binders, such as thermoplastic binder systems. Many attempts have been made to minimize or remove the excess carbon introduced by the organic binders.
Previously, an attempt to minimize the level of carbon introduced by organic binders in sintered tungsten carbide-containing materials was through the use of soluble binders which had to be removed using chemical extraction, prior to sintering. However, such chemical extraction required additional handling of the object, the use of hazardous solvents, and controlled drying conditions to maintain integrity of the object.
Another prior art attempt to minimize the level of carbon introduced by organic binders in sintered tungsten carbide-containing materials was by reacting the carbon with hydrogen gas during burn-out and/or sintering. However, the flammability of hydrogen gas presented additional safety concerns.
Yet another prior art attempt at minimizing the level of carbon introduced into sintered tungsten carbide-containing materials was through the use of alternative binder systems which left little or no residual carbon in the pre-sintered material after pyrolysis. However, the ceramics industry is familiar with traditional thermoplastic binder systems and their processing techniques, so that the development and utilization of alternative binder systems was seen as undesirable.
The sintering of a tungsten carbide-containing preform may be accomplished by pressure-assisted or pressureless processes. Pressure-assisted sintering processing is especially suitable for forming the most dense, hard tungsten carbide-containing materials. However, one of ordinary skill in the art knows that pressure-assisted sintering processes are expensive and difficult to use when forming objects of complex shapes or objects having small openings. Pressureless sintering, on the other hand, has been found extremely suitable for forming complex-shaped objects and objects with small openings.
It is, therefore, desirable to have an improved tungsten carbide (WC) based material with improved control of the total carbon therein as it results in a sintered tungsten carbide-containing material which has improved hardness, increased density, and may be prepared by simple methods which do not rely on using soluble binders and removing them thereafter, or on reacting with flammable hydrogen gas. It is also desirable to have available a method which is simple and capable of forming complex sintered objects that may have small openings.